1. Field of the Invention
The present invention relates to a rotary actuator which is composed of at least one magnet and at least one coil body and which performs a rotation operation with the use of magnetic force, and in particular relates to a rotary actuator which is incorporated in a camera lens and suitable for use as a drive source for an autofocus mechanism.
2. Description of the Related Art
DC motors are generally used as actuators for performing a rotation operation with the use of magnetic force. For instance, a DC motor is incorporated in an SLR camera lens to be used as a drive source for performing a lens driving operation when autofocus (AF) control is performed. For example, in Japanese Unexamined Patent Publication Nos. 2006-259113 and 2006-145783, a DC motor is incorporated in a camera body and another DC motor is incorporated in a photographic lens, and appropriate AF control is performed by appropriately switching operations of these DC motors. In addition, instead of using DC motors, the use of ultrasonic motors that can be designed small and light-weight as motors incorporated in a camera lens has been proposed in recent years. As this type of ultrasonic motor, the assignee of the present invention has proposed an ultrasonic motor that is disclosed in, e.g., Japanese Unexamined Patent Publication No. 2007-116757.
When AF control is performed in a photographic lens, normally a rotation of a focus cam ring relatively moves a focusing lens group linearly along the optical axis. A torque of a DC motor or an ultrasonic motor is used as a drive source for controlling rotation of this focus cam ring. However, in small DC motors and small ultrasonic motors, it is difficult to obtain a level of torque sufficient to (directly drive) rotate the focus cam ring, and accordingly, a reduction gear device is typically employed with the DC or ultrasonic motor so that torque of the motor is converted into high torque to be transferred to the focus cam ring via the reduction gear device. In addition, in the case of using an ultrasonic motor as a drive source (for controlling rotation of the focus cam ring), a high-voltage circuit is required to drive the ultrasonic motor; moreover, it is required to secure a space for installation of the high-voltage circuit. Therefore, the internal space of the photographic lens needs to accommodate both the reduction gear device and the high-voltage circuit, which has been an obstacle to miniaturization and a weight reduction of the photographic lens.